Polylactic acid (PLA) is a commercial biobased polymer that is known to be biodegradable in an industrial composting environment. The glass transition temperature of PLA is about 55-60° C. and it can be made highly crystalline depending on its stereochemistry and processing conditions. As a result of its high crystallinity, PLA has a tensile modulus of about 2-3 GPa, which is considerably higher than that of polyethylene and polypropylene. However, the toughness of PLA is generally considered to be quite low and inadequate for many applications.
There have been several attempts to increase the toughness of PLA by blending it with other additives and polymers. One of the suppliers of PLA (INGEO®), NatureWorks, found on the world wide web at “natureworksllc” which describes some of the polymer blend approaches used to increase the toughness of PLA. While some of the polymer blend approaches show promise, the options become very limited when the biodegradability of the blend components are taken into account.
A recent review article (K. S. Anderson, et al., Polymer Reviews, 48, 85 (2008)) described the various approaches employed to toughen PLA using different blend components. Some plasticizers such as citrate esters help increase the toughness of PLA; however, this increase in toughness comes at the expense of tensile strength which is decreased by a factor of about seven (L. V. Labrecque, et al., Journal of Applied Polymer Science, 66, 1507 (1997)). The use of low molecular weight polyethylene glycol (PEG) as a polymeric plasticizer also seemed to improve the toughness of PLA; however, the tensile strength was still compromised considerably although not to the extent of the citrate ester plasticizer (M. Baiardo, et al., Journal of Applied Polymer Science, 90, 1731 (2003)). Polycaprolactone (PCL) is a biodegradable aliphatic polyester that has been blended with PLA with some modest improvements noted in toughness depending on the nature of the blend preparation and the use of compatibilizing agents (L. Wang, et al., Polymer Degradation and Stability, 59, 161 (1998); M. E. Broz, et al., Biomaterials, 24, 4181 (2003); H. Tsuji and Y. Ikada, Journal of Applied Polymer Science, 60, 2367 (1996); M. Hiljanen-Vainio, et al., Macromolecular Chemistry and Physics, 197, 1503 (1996)). More recently, reactive blending of PLA with polyacrylic acid followed by physical blending of polyethylene glycol in solution have shown promise for increasing PLA toughness while maintaining modulus and tensile strength (R. M. Rasal, D. E. Hirt, Macromolecular Materials and Engineering, 295, Iss.3, 204 (2010)). However, this process only improved the toughness of PLA by approximately 10-fold.
The most significant improvement in the toughness of PLA was reported in U.S. Pat. No. 5,883,199 by McCarthy and co-workers. In this invention, blends of PLA with polybutylene-succinate adipate (PBSA) (BIONOLLE 3001 from Showa Highpolymer Co. Ltd, Japan) showed a remarkable improvement in tensile toughness and elongation relative to the PLA homopolymer. Specifically, a 70/30 (weight percent) PLA/PBSA blend showed about a 50-fold improvement in tensile elongation to break and about a 25-fold increase in tensile toughness. However, while PBSA is a known biodegradable polymer, it is currently synthesized from petrochemical starting materials and is therefore not biobased.
Polyhydroxyalkanoates (PHAs) are a unique material to address the PLA toughness issue because PHAs are easily blended with PLA, they are biobased and biodegradable in a number of different environments. PLA/PHA blends have been prepared and characterized by several research groups. These include blends of PLA with poly-3-hydroxybutrate (P3HB) homopolymers and poly-3-hydroxybutyrate-co-hydroxyvalerate (PHBV) copolymers which showed small improvements in PLA toughness at loading levels of about 10-30 weight percent (J. S. Yoon, W. S. Lee, K. S. Kim, I. J. Chin, M. N Kim and C. Kim, European Polymer Journal, 36, 435 (2000); B. M. P. Ferreira, C. A. C. Zavaglia and E. A. R. Duek, Journal of Applied Polymer Science, 86, 2898 (2002); I. Noda, M. M. Satkowski, A. E. Dowrey and C. Marcott, Macromolecular Bioscience, 4, 269 (2004); K. M. Schreck and M. A Hillmeyer, Journal of Biotechnology, 132, 287 (2007)). Noda and co-workers (I. Noda, M. M. Satkowski, A. E. Dowrey and C. Marcott, Macromolecular Bioscience, 4, 269 (2004)) noted a 7-fold increase in tensile toughness in PLA at a 20 weight percent loading of a poly-3-hydroxybutyrate-co-3-hydroxyhexanoate (P3HB-3HH) copolymer only for timescales for which the PHA remained non-crystalline. It should be noted that the timescale can be theoretically manipulated to be as long as 2-4 years depending on the size-scale of the dispersed PHA domains. While an 80/20 PLA/P3HB-3HH blend appears promising, others have had difficulty reproducing this result (K. M. Schreck and M. A. Hillmeyer, Journal of Biotechnology, 132, 287 (2007)).
Therefore, a need exists for producing blend compositions of polylactic acid and polyhyroxyalkanoates with improved reproducible mechanical properties for the overall composition.